This first fellowship is probably the most prestigious: it is the Graduate Research Fellowship Program(GRFP), and it is sponsered by your good friend the National Science Foundation (NSF). This Fellowship consists of three years of support during a five-year fellowship period. Currently, NSF provides a stipend of $34,000 to the Fellow and a cost-of-education allowance of $12,000 to the graduate degree-granting institution for each Fellow who uses the fellowship support in a fellowship year. Note that there is an application eligibility and a finite number of times you can apply. I believe the stipulation is this: you can apply once as an undegraduate heading into a graduate program, and a graudte student in his first year entering his second year.The deadline is late October and is different depedning on what major you choose. I applied for GRFP and I got an honorable mention, which that in itself has some perks. Also, there are plenty of resources online to help you with your application process.
The second fellowship is one that I am currently a part of: it is the Bridge to the Docotorate Program here at UCSB (UCSB BD Program). This fellowship is sponsored by the National Science Foundation Louis Stokes Alliance for Minority Participation (LSAMP) and is part of the California Alliance for Minority program (CAMP). Note that this program rotates UC's every year. Hence, you have to keep an eye out on where the program will be hosted the year you apply. The Fellowship consists of $32,000 annual stipend plus full tuition, fees, and health insurance for first two years. Guaranteed support for years 3-5 through Graduate Student Researcher or Teaching Assistantships. This fellowship also has some stipulations. First, you have to be a participant of the LSAMP program or CAMP program. Second, a mimimum of a 3.0 undergraduate GPA. Third, admission to a UCSB STEM graduate program. Lastly, be a U.S. Citizen or Permanent Resident.
This third fellowship is a little different than the two presented previously. The LSAMP-BD program is a fellowship for a masters at a California State University with the intention to achieve a doctorate in the future. This fellowship consists of a $32,000 stipend per year, cost of eduction allowance, mentorship, professioanl development, travel stipends and much more. Similar to the UC BD program, this program rotates every year from Cal States to Cal states. The past couple of years this program was at California State University of Los Angeles (CSULA LSAMP BD Cohort 13), in which I was a member. The link is from a previous year. I will make sure to upload the link once I know where the program is being held for the Fall 2020. The Fall 2019 Cohort for the LSAMP BD program is taking place in Fresno State University.
Note that there are many more BD programs across the country. These are the only ones I know that are in the state of California.
Another fellowship that I know of is the (Ford Foundation Fellowship Program). For now this is all I got.